Molten salt electrolytes for

Early in their work on molten salt electrolytes for thermal batteries, the Air Force Academy researchers surveyed the aluminium electroplating literature for electrolyte baths that might be suitable for a battery with an aluminium metal anode and chlorine cathode. They found a 1948 patent describing ionically conductive mixtures of AICI3 and 1-ethylpyridinium halides, mainly bromides [6]. Subsequently, the salt 1-butylpyridinium chloride/AlCl3 (another complicated pseudo-binary)... 

FIGURE 9.1. Coulometric titration curves for the systems Li-Sb and Li-Bi using a molten salt electrolyte for lithium ions. The equilibrium cell voltage is plotted with reference to elemental lithium as a function of the lithium concentration. 

In 1963, Major (Dr.) Lowell A. King (Figure 1.1) at the U.S. Air Force Academy initiated a research project aimed at finding a replacement for the LiCl/KCl molten salt electrolyte used in thermal batteries. 

For a review of salts formerly thought of as low-temperature ionic liquids, see Mamantov, G., Molten salt electrolytes in secondary batteries, in Materials for Advanced Batteries (Murphy, D. W., Broadhead, J., and Steele, B.C. H. eds.). Plenum Press, New York, 1980,... 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Attention has been given for some time to the use of lithium alloys as an alternative to elemental lithium. Groups working on batteries with molten salt electrolytes that operate at temperatures of 400-450 °C, well above the melting point of lithium, were especially interested in this possibility. Two major directions evolved. One involved the use of lithium-aluminium alloys [5, 6], whereas another was concerned with lithium-silicon alloys [7-9]. 

Because of the interest in its use in elevated-temperature molten salt electrolyte batteries, one of the first binary alloy systems studied in detail was the lithium-aluminium system. As shown in Fig. 1, the potential-composition behavior shows a long plateau between the lithium-saturated terminal solid solution and the intermediate P phase "LiAl", and a shorter one between the composition limits of the P and y phases, as well as composition-dependent values in the single-phase regions [35], This is as expected for a binary system with complete equilibrium. The potential of the first plateau varies linearly with temperature, as shown in Fig. 2. 

The lithium-silicon system has also been of interest for use in the negative electrodes of elevated-temperature molten salt electrolyte lithium batteries. A composition containing 44 wt.% Li, where Li/Si=3.18, has been used in commercial... 

In most metallurgical operations, either aqueous or molten salt electrolytes are used only very rarely may an organic electrolyte be selected. The selection of the most suitable electrolyte is based on a variety of considerations. The choice of an electrolyte for lead, for example, is guided by the facts that lead forms an insoluble sulfate if sulfuric acid is used, and that a peroxide of lead is formed in solutions of other mineral acids. An electrolyte of lead fluorosilicate in hydrofluorosilicic acid (H2SiF6) is used in order to circumvent these problems. 

Electrolytic cells are constructed of materials that can withstand the action of the electrolytes and of the electrode products. The cell may be of the open type or may be partially or fully closed, depending on the requirement of handling the electrode products. Some of these cells will be described while dealing with the production of specific metals. Very stringent requirements are imposed when considering the design of electrolytic cells for the deposition of refractory and reactive metals. Most of such metals are produced by using molten salt electrolytes. These metals are prone to atmospheric contamination at the electrolysis temperature, and it is thus necessary to operate the cell under an inert atmosphere. 

Electrorefining has been used for the purification of many common as well as reactive metals. It has been seen that the emf or the potential required for such a process is usually small because the energy needed for the reduction of the ionic species at the cathode is almost equal to that released by the oxidation of the crude metal at the anode. Some metals, such as copper, nickel, lead, silver, gold, etc., are refined by using aqueous electrolytes whereas molten salt electrolytes are necessary for the refining of reactive metals such as aluminum,... 

The availability of low-melting salt combinations opens up potentialities for all the light metals. As we have seen above, almost all the research work on RED SBs with molten salt electrolytes is carried out with Li as an anode and with chlorides as electrolytes. Other light metal systems should be investigated too, especially in view of the much greater natural abundance of some of these other metals. 

Fig. 3.10 Mott-Schottky plot for n-type and p-type semiconductor of GaAs in AlCls/n-butylpyridinium chloride molten-salt electrolyte [79],...

By analogy with the HaU-Heroult process for electro winning of aluminum, first attempts used molten salt electrolytes at high temperature [1]. Silicon oxide... 

This work attempts to model a semiconductor/molten salt electrolyte interphase, in the absence of illumination, in terms of its basic circuit elements. Measurement of the equivalent electrical properties has been achieved using a newly developed technique of automated admittance measurements and some progress has been made toward identification of the frequency dependent device components (1 ). The system chosen for studying the semiconductor/ molten salt interphase has the configuration n-GaAs/AlCl3 1-... 

Fleischmann et al s 34 report cyclic voltammetry data for the oxidation of a series of aromatic hydrocarbons in a molten salt electrolyte, AlCl3-NaCl-KCl at 150°. Electrooxidation in this medium occurs at unusually low oxidation potentials. Tris-(p-substituted phenyl)amines, with the exception of tri (p-nitrophenyl) amine, yield very stable radical cations by all electrochemical criteria 380>S42 Mono- and bis-p-substituted triphenylamines, however, dimerize with rate constants ranging from 101 to 10s M 1 sec 1 to benzidines 176 (Eq. (237)), which subsequently are oxidized to the radical cations 177, whose ESR-spectra are observed. Dimerization is fastest with the p-N02 andp-CN-derivative, in accordance with HMO calculations, which predict the highest spin sensity in the p-position of these compounds 542 ... 

Molten Carbonate (MCFC) These cells use a mixed alkali-carbonate molten salt electrolyte and operate at about 600 °C. They are being developed for continuously operating facilities, and can use coal-based or marine diesel fuels. 

Like fuel cells, batteries using molten salt electrolytes offer high performance. Molten salts have very high electrical conductivity, which permits the use of high current densities. Likewise, molten salts permit the use of highly reactive electrode materials, which cannot be used in aqueous electrolytes. For these reasons, batteries with molten salts offer very high specific energy (>100 Wh/kg). To... 

Electrorefining in aqueous media is extensively applied for the production of copper, nickel, lead, tin, cobalt, silver, and gold, while in molten salt electrolytes it is practically limited to aluminum. 

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